Analysis
Roserade successfully establishes it's niche in the December metagame as a threatening sacrifice lead, utility support and potential cleaner thanks to it's great role compression, multitude of useful moves, Grass/Poison typing, above average Speed and Special Attack stats and choice of ability between the Status-Absorbing Natural Cure or the punishing Poison point. Some remarkable examples of Roserade's capabilities include: spreading hazards with Spikes and Toxic Spikes, providing status support with Stun Spore, Sleep Powder, Toxic and Aromatherapy, access to STAB Sludge Bomb and Energy Ball, and, on top that, access to weather dependent recovery in Synthesis, valuable qualities few of December's chosen can replicate. In practice, it can wall and check Milotic and Primarina, and threaten big damage on Guzzlord, Ribombee, water-types, miscellaneous Pokémon and switch-ins with STAB Sludge Bomb and Energy Ball/Giga Drain, plus access to Dazzling Gleam and Shadow Ball. However, Roserade faces significant competition from Celebi and Ribombee thanks to their similar options and roles, can struggle to fit everything it wants to use on a single moveset and is checked by various Pokemons such as Celebi, Yanmega, Chandelure, Ice Types, Lokix, Magmortar, Physical hits and Psychic coverage, which limit how splashable, useful and longevous it can be. What truly sets Roserade apart from it's competition is it's unique access to both Sleep Powder and Entry Hazards, further cementing it's presence as a playmaking lead. If you can fit this Rose Pokemon in your team, it is almost guaranteed to do something useful.
Footnote: Dazzling Gleam is a recent development due to Okidogi's ban. Before Dogi's ban, Roserade always opted for Extrasensory which would consistently OHKO most Dogi sets, provided Roserade had sufficient SPA Investment and Dogi wasn't all in on SPD.
Written by Beps
Quality checked by Ocean, Ryu, & Typhon